The invention systems, and more particularly relates to proportional control systems. In its most immediate sense, the invention relates to proportional control systems wherein the sensor is well-coupled to the controlled variable.
In a thermostatic control system such as is used in many applications, a sensor turns e.g. a heater on when the sensor temperature is below the desired control temperature and turns the heater off when the sensor temperature rises above the desired temperature. An example of this type of system is a conventional electrical water heater.
It has long been known that such a thermostatic control system does not use energy efficiently. This is because the control temperature must as a practical matter be set higher than the desired temperature and energy is used to overshoot the desired temperature.
Therefore, in applications where energy efficiency is important, such as in heating equipment on an aircraft (where power resources are limited) proportional control systems have been used. In these, a proportioning band is established and the duty cycle of the heater is varied between 100% (full on) at the bottom of the band and 0% (full off) at the top of the band. This sort of control system regulates the temperature much more closely than does a thermostatic control system and consequently requires less power from the airplane.
In both sorts of systems, the sensor (e.g. the thermostat or thermistor) is poorly coupled to the heater. In practice, this usually means that the sensor is located far away from the heater, so that the sensor responds not to the temperature of the heater, but rather to the temperature of the heated material (e.g. water, windshield, breeze surface).
However, there are applications where it is impossible or highly difficult to space the sensor and heater far apart. For example, commercial aircraft have water lines which supply water to the lavatories and galleys. These lines may be exposed to ambient temperatures of -30.degree. F. and less. It is therefore necessary to heat the lines to prevent them from freezing up.
One of the best ways in which to do this is to insert (as through a tee fitting) a heating element into the water line. In such an application, it is sometimes impractical (because of space or other constraints) to use a separate sensor which is spaced from the heating element. In such an application, if a sensor is to be used at all, it must be placed immediately adjacent the heating element.
If such a heater is to be driven by a conventional proportional control circuit, the system will not operate properly. When the heater is off and the water temperature drops below 32.degree. F., the sensor will call for heat and the heater will turn on. This will immediately raise the temperature of the sensor above 32.degree. F. (without heating the water very much) and the sensor will call to turn the heater off. Then, the sensor will immediately cool down, will immediately call for heat, and the system will cycle on and off without heating the water. The water will therefore freeze.
It would be advantageous to provide a proportional control system which would operate properly even when the sensor is well-coupled to the heater.
One object of the invention is to provide a control circuit and system which permits proportional control of e.g. a heater even when the sensor which governs the operation of the heater is well-coupled to the heater.
Another object is, in general, to improve on known circuits and systems of this general type.
The invention proceeds from a realization that a proportional control circuit used in an application such as has been described above should ideally have a proportioning operation in which the duty cycle of the heater should increase with increasing sensor temperature instead of decreasing with increasing sensor temperature as is the case in known proportional controllers.
However, such an operation would seem to be completely impractical because it calls for positive feedback. Conventional analysis would rule such positive feedback out because it is unstable. As an example, if the heater turned on, the sensor would call for more heat. This would increase the temperature of the heater, increase the temperature of the sensor, and would keep on until the whole system burned out.
In accordance with the invention, a negative feedback loop and a positive feedback loop are superposed. The time constant of the negative feedback loop is less than the time constant of the positive feedback loop. As a result, the negative feedback characteristics predominate at all times and the system is therefore stable even though it has positive feedback characteristics.